1. Field of the Invention
The present invention relates to lifting devices and more particularly to a hydraulic lifting device for lifting a blowout preventer from a wellhead.
2. Brief Description of the Prior Art
Blowout preventers, referred to in the oil and gas industry as "BOPs," are used to prevent blowouts during the drilling and production of oil and gas wells. The BOP is installed at the wellhead for the purpose of preventing the escape of pressure in an annular space between the casing and drill pipe, or in an open hole during drilling and completion operations. During the drilling operation from a drilling platform, the BOP is located some distance below the drilling rig floor. The drilling platform includes a rotary table which is mounted within a circular opening in the floor. The rotary table is used to turn the drill stem and support the drilling assembly.
The BOP is mounted on top of the well casing through which the drill string passes. BOPs are massive structures, weighing m excess of 45 tons in some drilling operations. The BOPs extend from the top of the casing to within a short distance of the bottom of the drilling platform.
At various times during the drilling of a well, it may become necessary to lift the BOP from the casing to allow access to the interior of the casing. Typically, the BOP is lifted approximately 12-18 inches above the wellhead.
In the past, BOPs have been lifted by using a plurality of hydraulic jacks suspended from the bottom of the drilling platform. It has also been common in the past to mount pulley systems to the underside of the drilling platform or to install a plurality of come-alongs having a handle which must be manipulated by a worker standing on a catwalk. These practices are dangerous and often times result in an unstable suspension of a very heavy BOP.
U.S. Pat. No. 4,125,164 to Terry discloses a plurality of hydraulic rams, which are connected to a BOP and the underside of the rig floor. The hydraulic rams are operated by an operator a safe distance from the BOP through the use of a portable fluid-distributing valve and pump assembly.
U.S. Pat. No. 4,305,467 to Villines discloses an apparatus for lifting a BOP, which is positioned on top of the rig floor and above the rotary table of the drilling platform The apparatus includes a pair of winches with cables for lifting the BOP.
It is desirable to have a stack lifter for a blowout preventer which can be easily installed on top of the rig floor and which will vertically raise and lower the BOP. It is further desirable to have a stack lifter with sufficient capacity to raise and lower massive BOPs, and do this in a stable and upright manner. It is also desirable that the stack lifter be extremely safe and economical to use and install.